[docs] Double headings on every page

Hi, I'm hitting an oddity in one of my projects where the titles of all pages show up twice.

Example:

https://docs.openstack.org/project-deploy-guide/charm-deployment-guide/walla...

Source file is here:

https://opendev.org/openstack/charm-deployment-guide/src/branch/master/deplo...

Does anyone see what can be causing this? It appears to happen only for the current stable release ('wallaby') and 'latest'.

Thanks, Peter